The present invention relates to optical transmissions, and in particular, reducing inter-channel cross-talk among low frequency telemetry signals in a wave division multiplexing (WDM) system.
Wave division multiplexing (WDM) systems are used to transmit multiple channels independently of one another through an optical fiber. Each channel has a unique optical frequency. At a receiving device the transmitted channels are demultiplexed based on the particular optical frequencies associated with the individual channels.
A problem encountered in conventional WDM systems is inter-channel cross-talk among low frequency telemetry signals, that is, telemetry signals having a frequency of approximately less than 100 kHz. Inter-channel cross-talk occurs in the optical amplifier as a result of mixing of the low frequencies. Mixing occurs when one or more optical signals are used as pumps to provide gain in the optical amplifier. In particular, if the low frequency telemetry optical signal intensity modulates the optical data channel it effectively modulates the gain of the amplifier, thereby resulting in mixing of the low frequency signals carried by the various channels. Because of mixing of the low frequency telemetry signals, the optical channels can not be isolated by the demultiplexer at the receiving end.
It is therefore desirable to avoid low frequency intensity modulation of the data channels in order to reduce or prevent inter-channel cross-talk of low frequency telemetry signals.
The wave division multiplexing system in accordance with the present invention solves the above noted inter-channel cross-talk problems associated with intensity modulated low frequency telemetry signals in conventional WDM systems, by phase modulating the low frequency telemetry signals.
The wave division multiplexing system includes a plurality of transmitters. Each transmitter has an optical source, such as a laser, for generating an optical signal and a radio frequency signal generator for producing a radio frequency signal having a low frequency telemetry signal impressed thereon. An optical phase modulator receives the optical signal and the radio frequency signal carrying the low frequency telemetry signal, and generates a phase modulated output optical signal. The phase modulated output signal generated by each of a plurality of transmitters are combined using a multiplexer and amplified before being transmitted over a single optical fiber. The frequency of modulation and phase shift of the optical signal are preferably selected to substantially. avoid phase modulation/amplitude modulation (PM/AM) conversion in a dispersive filter. After being transmitted over the optical fiber, a demultiplexer isolates the optical channels of each respective transmitter which are transmitted to respective receivers. Each receiver includes a splitter for tapping off from each demultiplexed optical channel a second portion of power from a first portion of power. A data recovery device reestablishes the data information based on the first portion of power, while an interferometer receives the second portion of power and together with a detector recovers the low frequency telemetry signal for the optical channel of the respective transmitter.
In addition, the present invention is directed to a method for using the wave division multiplexing system described above. A radio frequency signal is generated and a low frequency telemetry signal is impressed thereon, thereby generating a radio frequency output signal. The optical signal of each transmitter is phase modulated using the respective radio frequency output signal to produce phase modulated output optical signals that are multiplexed into a single signal before being transmitted over an optical fiber. A demultiplexer receives the multiplexed signal and separates it into respective optical channels. Then the data information and low frequency telemetry signal for each particular frequency is recovered. Specifically, a portion of the power is split off into a first portion and a second portion. The data information is reestablished from the first portion power using a data recovery device, while the low frequency telemetry signal is reestablished using an interferometer and detector.